A microchannel plate is a key component of an image intensifier tube. Image intensifier tubes are employed for the purpose of amplifying a low intensity or non-visible radiational image of an object into a readily viewable image. Many industrial and military applications exist for such devices including enhancing the night vision of aviators, rendering night vision to persons who suffer from retinitis pigmentosa, more commonly known as night blindness and photographing astronomical bodies.
The general construction of a prior art image intensifier tube is exemplified in FIG. 1 which illustrates a Generation III (Gen III) image intensifier tube 10. Examples of GEN III image intensifier tubes can be found in U.S. Pat. No. 5,029,963 to Naselli, et al., entitled REPLACEMENT DEVICE FOR A DRIVER'S VIEWER and U.S. Pat. 5,084,780 to Phillips, entitled TELESCOPIC SIGHT FOR DAYLIGHT VIEWING both of which are manufactured by ITT Corporation, the assignee herein.
The GEN III image intensifier tube 10 shown in FIG. 1 comprises an evacuated envelope or vacuum housing 22 having a photo cathode 12 disposed at one end of the housing 22 and a phosphor screen 30 disposed at the other end of the housing 22. A microchannel plate (MCP) 24 is positioned within the vacuum housing 22 between the photo cathode 12 and the phosphor screen 30.
The photo cathode comprises a glass faceplate 14 coated on one side with an antiflection layer 16; a gallium aluminum arsenide (GaAlAs) window layer 17; a gallium arsenide (GaAs) active layer 18; and a negative electron affinity (NEA) coating 20.
The MCP 24 is located within the vacuum housing 22 and is separated from the photo cathode 12 by gap 34. An MCP is an electron multiplier formed by an array of microscopic channel electron multipliers. The MCP 24 is generally made from a thin wafer of glass having an array of microscopic channels extending between input and output surfaces 26 and 28 respectively. The wall of each channel is formed of a secondary emitting material. The phosphor screen 30 is located on a fiber optic element 31 and is separated from the output surface 28 of the MCP 24 by gap 36. The phosphor screen 30 generally includes aluminum overcoat 32 to stop light reflecting from the phosphor screen 30 from re-entering the device through the NEA coating 20.
In operation, infrared energy coming from an external object impinges upon the photo cathode 12 and is absorbed in the GaAs active layer 18, resulting in the generation of electron/hole pairs. The electrons generated by the photo cathode 12 are subsequently emitted into gap 34 of the vacuum housing 22 from the NEA coating 20 on the GaAs active layer 18. The electrons emitted by the photo cathode 12 are accelerated toward the input surface 26 of the MCP 24 by applying a potential applied across the input surface 26 of the MCP 24 and the photo cathode 12 of approximately 800 volts.
When an electron enters one of the channels of the MCP 24 at the input surface 26, a cascade of secondary electrons is produced from the channel wall by secondary emission. The cascade of secondary electrons are emitted from the channel at the output surface 28 of the MCP 24 and are accelerated across gap 36 toward the phosphor screen 30 to produce an intensified image. Each microscopic channel functions as a secondary emission electron multiplier having an electron gain of approximately several hundred. The electron gain is primarily controlled by applying a potential difference across the input and output surfaces of the MCP 24 of about 900 volts.
Electrons exiting the MCP 24 are accelerated across gap 36 toward the phosphor screen 30 by the potential difference applied between the output surface 28 of the MCP 24 and the phosphor screen 30. This potential difference is approximately 6000 volts. As the exiting electrons impinge upon the phosphor screen 30, many photons are produced per electron. The photons create an intensified output image on the output surface of the optical inverter or fiber optics element 31.
The image reproducing effectiveness of prior art MCPs depends in part on the ability of the cascading electrons coming from each channel of the MCP 24, to reach the phosphor screen 30 before any significant spatial dispersion occurs. If the cascading electrons spatially disperse before reaching the phosphor screen 30, the resolution of the intensified image will become degraded.
An additional problem results from the absorption of electrons impinging on the MCP surface from the photocathode. Referring again to FIG. 1, channels comprise approximately 60% of the top surface area of the MCP. Accordingly, the remaining 40% of the top surface of the MCP comprises a solid layer of glass which absorbs electrons incident from the photocathode. Therefore, approximately 40% of the electrons emitted from the photocathode are lost upon reaching the MCP.
The process of deposition of fissured material has been used in the manufacture of electron multipliers for use in particle counters and photo multipliers. Such devices are made by incorporating secondary emitting materials onto a wire mesh for supporting the device, and applying an electric field across the fissured material. In this manner, incoming particles or electrons are multiplied by producing secondary electrons which cascade through the fissures in the material in a manner similar to the channels of a microchannel plate. However, such an electron multiplier as described, while useful in non-imaging devices such as particle counters, could not be used in an image intensifier system because of the need for a mesh or similar supporting structure placed adjacent the MCP. Such a mesh would project onto the phosphor screen, and thus obfuscate the purpose of an image intensifier. Furthermore, a device of this type with sufficient thickness to provide the required electron gain is likely to have poor spatial resolution as well as poor modulation transfer function (MTF) performance as a result of lateral electron spreading during the multiplication process.
Other applications have used a solid material having secondary emission characteristics as an overcoat disposed on the MCP webbing or mesh, and that penetrated some distance into the MCP channel. While such a coating may provide first strike secondary emission for incoming electrons or particles, such devices fail to provide any cascade gain. Moreover, such coating does not serve as an ion barrier for blocking ions generated as a result of secondary emission and which travel back up the channel toward the photo cathode with potentially damaging effects.
Accordingly, it is desirable to obtain a device which provides secondary emission of particles or electrons incident to an MCP, resulting in a greater cascade gain and which acts as an ion barrier for protecting a photo cathode while providing proper spatial resolution for imaging.